Throw error if empty vector of states is given to Trajectory constructor in C++

[ysthakur]

Closes #4141.

[Starlight220]

Does this apply to Java as well?

[ysthakur]

Java doesn't segfault, but yeah, it'd be helpful there too. I'll wait to see if there's a legitimate use for empty states before committing, because there are no-arg constructors in both Java and C++ (@Starlight220).